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Rasterizer-ordered views

05/31/2018

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In this article

Rasterizer-ordered views (ROVs) allow pixel shader code to mark unordered access view (UAV) bindings with a declaration that alters the normal requirements for the order of graphics pipeline results for UAVs. This enables order-independent transparency (OIT) algorithms to work, which give much better rendering results when multiple transparent objects are in line with each other in a view.

Overview

Standard graphics pipelines may have trouble correctly compositing together multiple textures that contain transparency. Objects such as wire fences, smoke, fire, vegetation, and colored glass use transparency to get the desired effect. Problems arise when multiple textures that contain transparency are in line with each other (smoke in front of a fence in front of a glass building containing vegetation, as an example). Rasterizer-ordered views (ROVs) enable the underlying OIT algorithms to use features of the hardware to try to resolve the transparency order correctly. Transparency is handled by the pixel shader.

Rasterizer-ordered views (ROVs) allow pixel shader code to mark UAV bindings with a declaration that alters the normal requirements for the order of graphics pipeline results for UAVs.

ROVs guarantee the order of UAV accesses for any pair of overlapping pixel shader invocations. In this case “overlapping” means that the invocations are generated by the same draw calls and share the same pixel coordinate when in pixel-frequency execution mode, and the same pixel and sample coordinate in sample-frequency mode.

The order in which overlapping ROV accesses of pixel shader invocations are executed is identical to the order in which the geometry is submitted. This means that, for overlapping pixel shader invocations, ROV writes performed by a pixel shader invocation must be available to be read by a subsequent invocation and must not affect reads by a previous invocation. ROV reads performed by a pixel shader invocation must reflect writes by a previous invocation and must not reflect writes by a subsequent invocation. This is important for UAVs because they are explicitly omitted from the output-invariance guarantees normally set by the fixed order of graphics pipeline results.

Implementation details

Rasterizer-ordered views (ROVs) are declared with the following new High Level Shader Language (HLSL) objects, and are only available to the pixel shader:

RasterizerOrderedBuffer

RasterizerOrderedByteAddressBuffer

RasterizerOrderedStructuredBuffer

RasterizerOrderedTexture1D

RasterizerOrderedTexture1DArray

RasterizerOrderedTexture2D

RasterizerOrderedTexture2DArray

RasterizerOrderedTexture3D

Use these objects in the same manner as other UAV objects (such as RWBuffer etc.).

API summary

ROVs are an HLSL-only construct that applies different behavior semantics to UAVs. All APIs relevant to UAVs are also relevant to ROVs. Note that the following method, structures, and helper class reference the rasterizer: